1. Field of the Invention
The present invention relates to a circuit configuration for an electric machine, e.g., a starter motor for starting an internal combustion engine in a motor vehicle, including a current limiting device for limiting a starting current of the electric machine, and the present invention also relates to a method for operating a circuit configuration of this type.
2. Description of Related Art
Starting devices which are configured to have a starter motor according to the principle of a direct-current machine which uses mechanical commutation, in particular as a rotating-armature machine, are used for starting internal combustion engines in motor vehicles.
To achieve a necessary high torque and a high power of the starter at a comparatively low vehicle electrical system voltage of approximately 12 V to 14 V, or 24 V to 28 V, the direct-current machine is equipped with a small number of windings and a large line cross section. Due to the resulting low impedance, i.e., that of a rotor or also of a stator depending on the specific embodiment, the power loss is reduced and a high torque output is achieved at a comparatively good efficiency. On the other hand, it is the low impedance and, in particular, the low ohmic resistance, as well as the low inductivity, which result in high starting currents and a temporary voltage drop in a battery-operated vehicle electrical system. Only when the starter motor starts up is a voltage induced as the rotational speed increases, which counteracts the voltage drop. Additional consumers, which are connected to other points on the vehicle electrical system, must be designed for this voltage drop, or they must be inactive at the time of the voltage drop.
In conventional starting systems, the voltage drop during the startup is usually not critical, since additional consumers are largely not yet active or are inactive. In starter-based start-stop systems, in which the internal combustion engine is started by the starter motor even if the additional consumers are active, for example after a short stopping phase of the motor vehicle, ever greater demands are placed on a voltage stability function within the vehicle electrical system.
Published German patent application document DE 103 17 466 A1 describes a circuit configuration which has a starter motor for an internal combustion engine of a motor vehicle and which has a series resistor which is series-connected to the starter motor, namely an NTC series resistor, whose resistance value decreases as the temperature increases. When the starter motor starts up, the NTC series resistor heats up and its resistance value decreases.
Published German patent application document DE 102 31 088 A1 describes a circuit configuration of a starting device for an internal combustion engine of a motor vehicle which has two series-connected transistors, two resistors being connected in parallel to one transistor in each case as a voltage divider to diagnose a short-circuit fault of the transistors in a deactivated state of the transistors.